1. Field of the Invention
The present invention relates to a socket for measuring a semiconductor device received in a package of Ball Grid Array structure (a package having a substrate on the surface of which a semiconductor device is mounted and on the opposite surface of which solder balls are formed in the form of grating). More particularly, the present invention relates to a socket for a Ball Grid Array structure package (hereinafter referred to as BGA package) which is applied to a semiconductor integrated circuit testing apparatus (commonly called an IC tester) for testing a semiconductor integrated circuit (IC) in order to electrically connect between the solder balls of the BGA package which are terminals or electrodes thereof and the IC tester side, though the present invention is not limited thereto.
2. Description of the Related Art
With ICs formed in high density, many leads are required for leading out the internal circuits of an IC, and it is difficult to arrange these many leads in straight lines on a package in which the IC is accommodated because the leads are too much to put thereon.
From the above reason, recently, as shown in FIGS. 5(a) and 5(b), there is being put to practical use an IC package 10 having a substrate 13 made of, for example, a ceramic or plastic material in which an IC (not shown) is mounted on the surface of the substrate 13 and minute balls 12 each made of a solder functioning as terminals or electrodes are two-dimensionally arranged in the form of a matrix on the bottom surface of the substrate 13. Such IC package is called a BGA package as stated above. The internal circuits, i.e., IC of this BGA package are sealed by a mold denoted by a reference numeral 11 and the wiring of the IC is electrically connected to the solder balls 12 via printed circuits formed on the substrate 13.
In order to test the BGA package 10 mentioned above by the IC tester, it is necessary to electrically connect between the solder balls 12 which are the terminals or the electrodes of the BGA package 10 and the IC tester side. This electrical connection is achieved by interposing a socket for measuring a BGA package between the BGA package 10 and the IC tester.
An example of a conventional BGA package measuring socket is shown in FIG. 6(a). This BGA package measuring socket as generally indicated by a reference numeral 30 is mounted on a printed circuit board 20 which is a part of the IC tester side. This printed circuit board 20 has apertures (through-holes) 21 pierced at the positions thereof corresponding to the respective solder balls 12 arranged in a matrix on the bottom surface of the BGA package 10. A terminal portion 34.sub.3 of each of contact pins 34, described later in detail, is inserted into each of the through-holes 21. Therefore, the solder balls 12 of the BGA package 10 are electrically interconnected to a measuring circuit of the IC tester via the contact pins 34 of the BGA package measuring socket 30, respectively.
The BGA package measuring socket 30 includes a housing 31. This housing 31 is formed by working a thick plate-like body of an insulating material and has a mounting recess 32 formed on the upper surface of the housing 31, on which a BGA package 10 is mounted for testing. In addition, many pockets 33 extending from the position of the mounting recess 32 to the proximity of the bottom of the housing 31 are formed in the housing. These pockets 33 are provided to accommodate the contact pins 34 therein, respectively. Therefore, these pockets 33 are formed at the positions of the housing corresponding to the solder balls 12 arranged in a matrix on the bottom surface of the BGA package 10 mounted on the mounting recess 32 and are arranged in the form of a matrix as with the solder balls 12.
Each of the contact pins 34 is fabricated from a sheet metal having a resilience such as beryllium copper, phosphor bronze or the like by forming a strip from such sheet metal as the raw material and bending and deforming the strip by press working so as to have a vertical section substantially like a numeral "7" as can be understood from FIG. 6(a). In this press working, as shown in FIG. 6(b) in enlarged size, the contact portion 34.sub.1 of the top of the contact pin 34 to be brought into contact with the corresponding solder ball 12 is formed into a flat surface of an approximately regular square at the central portion of which a circular opening is formed. This circular opening is smaller in diameter than the solder ball 12. In addition, a portion of the contact pin 34 adjacent to the contact portion 34.sub.1 is formed as a curved portion 34.sub.2 for giving an elasticity in up-and-down direction to the contact pin 34 and extends in substantially vertical direction from the contact portion 34.sub.1 to the bottom of the housing. A flat rectangular plate-shaped base is formed in vertical direction at the lower side of the curved portion 34.sub.2 and is fitted into a hole or slit formed on the bottom of the housing. A pin-shaped terminal portion 34.sub.3 pending downwardly from the plate-shaped base is formed integrally with the plate-shaped base. As mentioned above, this pin-shaped terminal portion 34.sub.3 is inserted into the corresponding through-hole 21 pierced through the printed circuit board 20.
Each contact pin 34 thus construcetd is fixed to the housing 31 of the BGA package measuring socket 30 by fitting and fixing the plate-shaped base of the contact pin 34 positioned above the terminal portion 34.sub.3 thereof into the corresponding hole formed on the bottom of the housing 31.
The BGA package measuring socket 30 is placed on the printed circuit board 20 which constitutes a part of the IC tester side in the state that the tip of the terminal portion 34.sub.3 of each contact pin 34 is inserted into the through-hole 21 of the printed circuit board 20. Then, as shown in the drawing, the tip of the terminal portion 34.sub.3 of each contact pin 34 and a printed circuit formed around the through-hole 21 are soldered with each other on the back surface of the printed circuit board 20. By this process, the BGA package measuring socket 30 is fixed on the printed circuit board 20 and is electrically connected to the IC tester.
In such state, when a BGA package 10 to be measured is loaded onto the mounting recess 32 of the BGA package measuring socket 30 and is maintained under the condition that a downward pressure is applied thereto, each of the solder balls 12 on the under side of the BGA package 10 is engaged and held in pressure contact with the circular hole of the contact portion 34.sub.1 of each contact pin 34 of the BGA package measuring socket 30. This pressure contact force is generated by a reaction of the curved portion 34.sub.2 of the contact pin 34 caused by the downward pressure applied to the BGA package 10. In such a way, when each of the solder balls 12 on the back surface of the BGA package 10 is in pressure contact with the corresponding contact portion 34.sub.1 of each of the contact pins 34 of the BGA package measuring socket 30, the BGA package 10 is electrically connected to the IC tester side and a test for the BGA package 10 can be carried out.
The conventional BGA package measuring socket 30 uses the contact pin 34 as mentioned above. In this socket 30, the pressure contact force caused between each solder ball 12 on the under surface of the BGA package 10 and the contact portion 34.sub.1 of each contact pin 34 is generated by the curved portion 34.sub.2 of each contact pin 34 as discussed above, and therefore, in order to obtain a required pressure contact force, the length of the curved portion 34.sub.2 cannot be made shorter than a certain critical value. That is, in order to ensure the required pressure contact force, the entire length of the contact pin 34 is obliged to become relatively longer. The longer the entire length of the contact pin 34 becomes the higher the height of the socket housing 31 becomes, and hence the degree of the curvature of the curved portion 34.sub.2 becomes larger, resulting in that the dimension or size of the contact pin is increased in the direction perpendicular to the length-wise direction of the contact pin. Consequently, the packaging density of the contact pin 34 is lowered. Moreover, due to the increased entire length of the contact pin 34, various restrictions are added to the shape and structure of the housing 31, and thus the manufacturing cost is increased. Further, since the floating inductance and the floating capacitance are increased as the entire length of the contact pin 34 is increased, the longer length of the contact pin is also not desirable from the electrical view point. For example, it is difficult that a test for a BGA package at high frequency such as 100 MHz or so is carried out.
Another example of the conventional BGA package measuring socked is shown in FIGS. 7(a) and 7(b). This BGA package measuring socket 30 is almost the same as the prior art example shown in FIG. 6 concerning the shape and structure of the housing 31. That is, this housing 31 is also formed by working a thick plate-like body of an insulating material. On the upper surface of the housing is formed a mounting recess 32 on which a BGA package 10 is mounted for testing. In addition, many pockets 33 extending vertically are formed in the housing for accommodating probe pins 35 described later therein. These pockets 33 are through-holes passing through from the upper surface of the housing to the under surface thereof in this example, and are formed at the positions of the housing corresponding to the solder balls 12 arranged in a matrix on the bottom surface of the BGA package 10 mounted on the mounting recess 32 and arranged in the form of a matrix as with the solder balls 12.
Each probe pin 35 includes a cylindrical tube 35.sub.1 made of a conductive metal as shown in section of the probe pin 35 in FIG. 7(b). The upper end and the lower end of the tube 35.sub.1 are both closed and a circular hole 35.sub.2 of small diameter is formed at the center of each of the closed ends. Within the tube 35.sub.1 are housed disk-shaped bases of a pair of contacts 35.sub.3 each made of a conductive metal. A contact pin (35.sub.3) standing upright from each of the bases is led out through each of the circular holes 35.sub.2. A compressed coil spring 35.sub.4 made of a conductive metal having an elasticity is interposed between the bases of both contacts 35.sub.3 under compressed condition. Accordingly, both contacts 35.sub.3 are always pushed toward outside of the tube by the pressure force of the coil spring 35.sub.4.
In this case, the inner diameter of the cylindrical tube 35.sub.1 and the outer diameter of the disk-shaped base of each contact 35.sub.3 are selected so that the peripheral surface of the base of the contact 35.sub.3 can move up and down as it slides on and in electrical contact with the inner surface of the tube 35.sub.1. The diameter of the contact pin of the contact 35.sub.3 is also selected so that the contact pin can freely move through the small circular hole 35.sub.2 formed in the closed end. In case of practically assembling the above-stated probe pin 35, both the contacts 35.sub.3 with the coil spring 35.sub.4 between their bases are inserted into the tube 35.sub.1 whose upper end and lower end are open. Then, the upper and the lower ends of the tube 35.sub.1 are closed by plastic working so that the disk shaped bases of both the contacts 35.sub.3 and the coil spring 35.sub.4 are confined in the tube with the coil spring 35.sub.4 compressed between those bases. However, each of the circular holes 35.sub.2 of small diameter formed in the closed ends at the center thereof is left as it is.
The BGA package measuring socket 30 is placed on the printed circuit board 20 which forms a part of the IC tester side and positioned thereon. For this positioning, the tip of the contact pin of the lower contact 35.sub.3 of the probe pin 35 is contacted with a predetermined printed circuit portion on the printed circuit board 20 of the IC tester side. In this state, the BGA package 10 is loaded on the mounting recess 32 of the BGA package measuring socket 30. Then, a downward pressure is applied to the BGA package 10 and is kept thereon. Then, the solder ball 12 on the back surface of the BGA package 10 is kept in the state of pressure contact with the tip of the contact pin of the upper contact 35.sub.3 of the probe pin 35. The force for keeping the pressure contact is generated by the reaction of the coil spring 35.sub.4 of the probe pin 35 against the downward pressure applied to the BGA package 10. In such a way, the BGA package 10 is electrically connected to the IC tester side by pressure contacting the solder ball 12 on the back surface of the BGA package 10 with the tip of the contact pin of the probe pin 35 of the BGA package measuring socket 30. And thus, the test of the BGA package 10 can be carried out.
The BGA package measuring socket 30 shows in FIG. 7 uses the probe pin 35 as mentioned above. In this socket 30, the pressure contact force between the solder ball 12 on the back surface of the BGA package 10 and the tip of the contact pin of the upper contact 35.sub.3 of the probe pin 35, and the pressure contact force between the predetermined wiring portion of the printed circuit board 20 of the IC tester side and the tip of the contact pin of the lower contact 35.sub.3 are generated by the coil spring 35.sub.4 of the probe pin 35. Therefore, in order to obtain the required pressure contact force, the length of the coil spring 35.sub.4 cannot be made shorter than a certain threshold length. Therefore, in order to guarantee the required pressure contact force, the entire length of the coil spring 35.sub.4 is made relatively longer. The longer entire length of the coil spring 35.sub.4 increases the floating inductance and the floating capacitance. This is not desirable from electrical view point. For example, the test at high frequency of about 100 MHz is difficult to be carried out. In addition, the probe pin 35 comprises a cylindrical tube 35.sub.1 whose diameter is equal to or less than 1 mm. If the entire length of the probe pin 35 is extended, various constraints are added to the shape and the structure of the components of the probe pin 35. Thus, the manufacturing cost is increased.
Furthermore, as shown in FIG. 7(b), the conductive path within the probe pin 35 runs from the tip of the contact pin of the upper contact 35.sub.3 to the contact portion between the circumference surface of the base portion and the inner wall of the tube 35.sub.1 via the contact pin. The conductive path further runs from this contact portion to the other contact portion between the circumference surface of the base portion of the lower contact 35.sub.3 and the inner wall of the tube 35.sub.1 via the cylindrical tube, and further from this contact portion to the tip of the contact pin of the lower contact 35.sub.3. Therefore, there are four contact portions on the conductive path of the probe pin 35. That is, a contact between the solder ball 12 and the tip of the contact pin of the upper contact 35.sub.3, a contact between the circumference surface of the base portion of the upper contact 35.sub.3 and the inner wall of the cylindrical tube 35.sub.1, a contact between the inner wall of the cylindrical tube 35.sub.1 and the circumference surface of the base portion of the lower contact 35.sub.3, and a contact between the tip of the contact pin of the lower contact 35.sub.3 and the predetermined printed wiring portion of the printed circuit board 20 of the IC tester side are the four contact portions.
Further, since the contact states of those four contact portions are not necessarily good enough, the contact states are not stable. Therefore, a conductive path from the upper contact 35.sub.3 to the lower contact 35.sub.3 via the soil spring 35.sub.4 is sometimes formed instead of the conductive path mentioned above.